An Ikerbasque researcher at the University of the Basque Country disentangles the strange behaviour of qubits

Current technology enables the building of electrical circuits similar to those we use at home but reduced thousands of times in size to a micrometric scale of thousandths of a millimetre. When these circuits are built of superconductor materials and at near-absolute zero cryogenic temperatures, the world of everyday physics is left behind and the amazing world of quantum physics is entered. In this circuit the behaviour is something like an artificial atom (i.e. like the so-called quantum bits (“qubits”) of quantum computers) and the concepts of quantum optics, quantum information and condensed matter are mixed. An Ikerbasque researcher, ascribed to the University of the Basque Country (UPV/EHU), Enrique Solano, together with colleagues from Germany and Japan, have been working on an experiment and a theoretical model that show that certain quantum leaps are prohibited at times between levels of a qubit superconductor. This phenomenon is produced on sending photons of light with sufficient energy against a qubit installed within a circuit that simulates the behaviour of microwaves, similar to the ovens commonly used domestically but at a micrometric scale. The research has been published in the prestigious Nature Physics journal under the title, ‘Two-photon probe of the Jaynes-Cummings model and Controlled Symmetry Breaking in Circuit QED’. The article may be consulted on-line and will be included in the next print issue of the journal.

To explain this in an easy way, let us go back to our household circuit where, as with any such circuit, sufficient energy has to be supplied in order to move electrons from one place to another, i.e. the required voltage has to be applied. In an atomic circuit, however, the required energy is supplied through photons of light but this is not sufficient to produce the famous quantum jumps between two atomic energy levels. The additional required factor is the presence of the symmetry of the qubit, an enhancing factor, as it were. It is as if it were not enough for the quantum nature to have the required energy and it requires, moreover, the presence of the qubit to enable – or otherwise – the quantum leaps stimulated by the photons of light energy. If the qubit presents itself with symmetrical potential, the jump is prohibited and is not produced; curiously, if the potential is asymmetric, the quantum leap is permitted. This strange behaviour has been demonstrated by these researchers both at a theoretical level and in the laboratory, where the rules of prohibition may be activated and deactivated at will.

This research is an important step in the thorough understanding of the quantum jumps permitted and prohibited in superconductor circuits, as well as in the potential application of the electrodynamic quantum of circuits to future technology in quantum computation and information.

Enrique Solano is a PhD in Physics from the Federal University of Río de Janeiro. After working at the Ludwig-Maximilian University in Munich, he has been carrying out his research over the last few months at the UPV/EHU thanks to an agreement between the University and the Ikerbasque Foundation.